1. Field of the Invention
The present invention relates to a ballast for a pair of gaseous discharge lamps, and more particularly, to a fluorescent lamp ballast having means for reducing leakage current shock hazard and for minimizing EMI transfer from the load side to the line side.
2. Description of the Prior Art
U.S. Pat. No. 3,906,243 -- Herzog, assigned to the assignee of the present invention, discloses the concept of a retrofit type of emergency/normal fluorescent lighting system wherein some problems associated with the incorporation in the same enclosure of a high frequency battery driven oscillator and a power frequency ballast were solved. However, one difficulty found with this approach is that of low lamp light output in the normal line AC or power frequency mode. The ballast disclosed in the U.S. Pat. No. 3,906,243 -- Herzog, is of the lag type and with the size and temperature rise constraints imposed by the package, is able to deliver only about 40 - 45% of the rated light output of the fluorescent lamp. With this lag ballast design, current in the lamp circuit is that associated strictly with the lamp, being limited by the leakage reactance or self-inductance of the ballast transformer secondary winding. The primary winding is loosely coupled to the secondary and the current reflected in this primary winding is that associated with lamp load; however, since the power factor is approximately 50%, the current in the primary approaches double that normally expected. The Herzog U.S. Pat. No. 3,906,243 shows a way of coupling the high frequency oscillator into the system so that stray capacitance associated with the secondary winding exists between any lamp terminal and ground; such limits to safe values the leakage current which may be intercepted by a careless worker.
To improve power factor and to minimize volt-amperes in a ballast for operating a pair of gaseous discharge lamps, a lead circuit secondary may be employed to yield an approximately unity power factor in the primary, thereby reducing primary current by almost a factor of two and therefore primary size and heating by something approaching a factor of three to four.
A typical prior art circuit is shown in FIG. 1 for such a lead secondary ballast for operating a lamp from AC line voltage in conjunction with a high frequency oscillator, inverter 50, including a DC source for operating the lamp at a high frequency upon failure of the AC source. Such a circuit is non-symmetrical but isolated as seen in the figure. This type of circuit arrangement solves the power frequency ballasting light output problem but places all the stray capacitance of the power capacitor 40 on one ballast lead. If the high frequency oscillator 50 is attached as shown and is operating, a person grabbing terminal B when the lamp 12' has been removed from the fixture may complete a path to the high frequency oscillator through stray capacitance associated with the power capacitance 40 to the ballast case (not shown) and back to terminal A. The effect is that worst case leakage to ground may exceed double the maximum allowed by UL standards. Furthermore, this isolating secondary circuit arrangement also allows the transfer of high frequency noise, EMI, which is, for example, generated by a lamp, to the AC line or to ground. Such noise coupling results due to the power capacitor acting like a short circuit at high frequencies because of its position in the circuit and its associated relatively high capacitance to ground.
It is desirable, therefore, to provide a ballast for operating at least one gaseous discharge lamp wherein the leakage current to ground is reduced and transference to the ballast line side and to ground of EMI generated by the at least one gaseous discharge lamp is minimized.
Accordingly, it is an object of the present invention to provide a ballast for operating at least one gaseous discharge lamp from AC line voltage wherein means are provided for reducing the shock hazard resulting from leakage current in the ballast and for minimizing the transfer of high frequency interference from the load side of the ballast to the AC line side and to ground.